Semiconductor Inspection And Metrology System Using Laser Pulse Multiplier

ABSTRACT

A pulse multiplier includes a polarizing beam splitter, a wave plate, and a set of mirrors. The polarizing beam splitter receives an input laser pulse. The wave plate receives light from the polarized beam splitter and generates a first set of pulses and a second set of pulses. The first set of pulses has a different polarization than the second set of pulses. The polarizing beam splitter, the wave plate, and the set of mirrors create a ring cavity. The polarizing beam splitter transmits the first set of pulses as an output of the pulse multiplier and reflects the second set of pulses into the ring cavity. This pulse multiplier can inexpensively reduce the peak power per pulse while increasing the number of pulses per second with minimal total power loss.

RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent Application61/496,446, entitled “Optical Peak Power Reduction Of Laser Pulses AndSemiconductor Inspection And Metrology Systems Using Same” filed Jun.13, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to using optical peak power reduction oflaser pulses for semiconductor inspection and metrology systems, and inparticular to using a polarizing beam splitter and a wave plate togenerate an optimized pulse multiplier.

2. Related Art

The illumination needs for inspection and metrology are generally bestmet by continuous wave (CW) light sources. A CW light source has aconstant power level, which allows for images or data to be acquiredcontinuously. However, at many wavelengths of interest, particularly UVwavelengths, CW light sources of sufficient radiance (power per unitarea per unit solid angle) are not available.

A pulsed light source has an instantaneous peak power level much higherthan the time-averaged power level of a CW light source. However, if apulsed laser is the only available, or cost-effective, light source withsufficient time-averaged radiance at the wavelength of interest, thenusing a laser with the highest possible repetition rate and greatestpulse width is optimal. The higher the pulse repetition rate, the lowerthe instantaneous peak power per pulse for the same time-averaged powerlevel. The lower peak power of the laser pulses results in less damageto the optics and to the wafer being measured, as most damage mechanismsare non-linear and depend more strongly on peak power rather than onaverage power.

An additional advantage of an increased repetition rate is that morepulses are collected per data acquisition or per pixel leading to betteraveraging of the pulse-to-pulse variations and better signal-to-noiseratios. Furthermore, for a rapidly moving sample, a higher pulse ratemay lead to a better sampling of the sample position as a function oftime, as the distance moved between each pulse is smaller.

The repetition rate of a laser subsystem can be increased by improvingthe laser medium, the pump system, and/or its driving electronics.Unfortunately, modifying a ultraviolet (UV) laser that is alreadyoperating at a predetermined repetition rate can require a significantinvestment of time and money to improve one or more of its constituentelements, which may only incrementally improve the repetition rate.

Therefore, a need arises for a practical, inexpensive technique toimprove the repetition rate of a laser.

SUMMARY OF THE INVENTION

In general, a method of generating optimized pulses for a system isdescribed. In this method, an input laser pulse can be optically splitinto a plurality of pulses using a ring cavity. The plurality of pulsescan be grouped into pulse trains, wherein the pulse trains are ofapproximately equal energy and are approximately equally spaced in time.A set of the pulse trains can be transmitted as the pulses for thesystem, whereas a remainder of the pulse trains can be reflected backinto the ring cavity.

A pulse multiplier can include a polarizing beam splitter, a wave plate,and a set of mirrors. The polarizing beam splitter receives an inputlaser pulse. The wave plate receives light from the polarized beamsplitter and generates first and second sets of pulses. In oneembodiment, the wave plate includes a half-wave plate, which can be setat 27.3678 degrees. In another embodiment, the wave includes aquarter-wave plate. Notably, the first set of pulses has a differentpolarization than the second set of pulses. The set of mirrors createthe ring cavity, which includes the polarizing beam splitter and thewave plate. The polarizing beam splitter advantageously transmits thefirst set of pulses as an output of the pulse multiplier and reflectsthe second set of pulses back into the ring cavity.

The pulse multiplier can further include one or more lens for uniformlyshaping the pulses in the ring cavity. In one embodiment, a plurality oflenses can be implemented with two image relay tubes.

In one embodiment, the mirror set can include a composite mirror. Inanother embodiment, the mirror set can create two ring cavities thatshare the polarizing beam splitter and the wave plate. In yet anotherembodiment, the mirror set can create two ring cavities connected inseries, wherein each ring cavity includes its own polarizing beamsplitter and wave plate.

Another embodiment of a pulse multiplier without a ring cavity isdescribed. In this pulse multiplier, the polarizing beam splitterreceives an input laser pulse and the wave plate (e.g. a quarter-waveplate) receives light from the polarizing beam splitter and generates afirst set of pulses and a second set of pulses, the first set of pulseshaving a different polarization than the second set of pulses. A set ofmulti-surface reflecting components (e.g. a mirror and etalons) reflectsthe first and second sets of pulses back through the wave plate to thepolarizing beam splitter. The polarizing beam splitter transmits thefirst set of pulses as an output of the pulse multiplier and reflectsthe second set of pulses back to the wave plate and the set ofmulti-surface reflecting components. The peak output power of the secondset of pulses can be tunable to sin² θ.

Yet another embodiment of a pulse multiplier without a ring cavity isdescribed. In this pulse multiplier, a first wave plate receives aninput laser pulse and a polarizing beam splitter receives outputs of thefirst wave plate. A second wave plate receives a first set of pulsesfrom the polarizing beam splitter. A first mirror reflects outputs fromthe second wave plate back through the second wave plate to thepolarizing beam splitter. A third wave plate receives a second set ofpulses from the polarizing beam splitter. A second mirror reflectsoutputs from the third wave plate back through the third wave plate tothe polarizing beam splitter. Notably, the polarizing beam splittertransmits a third set of pulses from the second wave plate combined witha fourth set of pulses from the third wave plate to generate an outputof the pulse multiplier. The polarizing beam splitter also reflects afifth set of pulses from the second wave plate back to the second waveplate and the first mirror, and reflects a sixth set of pulses back tothe third wave plate and the second mirror. In one embodiment, the firstwave plate includes a half-wave plate, and the second and third waveplates include quarter-wave plates.

Any of the above-described pulse multipliers can be included in a waferinspection system, a patterned wafer system, a mask inspection system,or a metrology system. The pulse multiplier can inexpensively reduce thepeak power per pulse while increasing the number of pulses per secondwith minimal total power loss. The pulse multiplier can advantageouslyenable high speed inspection and metrology with off-the-shelf lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary pulse multiplier configured to generatepulse trains from each input laser pulse.

FIG. 2A illustrates exemplary energy envelopes output by the pulsemultiplier of FIG. 1. Each energy envelope includes an output pulsetrain.

FIG. 2B illustrates that the pulse multiplier of FIG. 1 can double theoriginal repetition pulse rate while reducing peak power and ensuringenergy balancing outputs.

FIGS. 3A, 3B, and 3C illustrate lens configurations in a pulsemultiplier for 1 lens, 2 lenses, and 4 lenses, respectively.

FIGS. 4 and 5 illustrate how mirror tilt can affect output beams offset.

FIG. 6 illustrates how lens tilt can affect output beams offset.

FIG. 7 illustrates how lens decenter misalignment can affect outputbeams offset.

FIG. 8 illustrates an exemplary embodiment of a pulse multiplierincluding two lenses.

FIG. 9A illustrates a pulse multiplier including two adjacent ringcavities connected in series.

FIG. 9B illustrates a pulse multiplier including a semi-nested ringcavity, thereby allowing the sharing of some components between two ringcavities.

FIG. 10 illustrates a pulse multiplier including multi-surfacereflection components.

FIG. 11 illustrates an exemplary pulse multiplier that uses two combinedbeams to generate pulse outputs.

FIG. 12 illustrates an exemplary pulse multiplier that reduces thenumber of mirrors in the ring cavity compared to the pulse multiplier ofFIG. 1.

FIG. 13 illustrates an exemplary wafer inspection system including apulse multiplier.

FIG. 14 illustrates an exemplary patterned wafer inspection systemincluding a pulse multiplier.

FIG. 15 illustrates another exemplary pulse multiplier.

FIG. 16 illustrates a ring cavity that can be implemented using onlyreflective optics.

FIG. 17 illustrates another exemplary pulse multiplier.

FIG. 18 illustrates another exemplary pulse multiplier.

DETAILED DESCRIPTION OF THE DRAWINGS

In accordance with one aspect of an improved pulse multiplier, eachlaser pulse can be optically split into a plurality of pulses, which aregrouped into pulse trains. In one embodiment, these pulse trains may beof approximately equal energy and may be approximately equally spaced intime. This splitting of the laser pulse can provide a practical andinexpensive solution to the above-noted problems with minimal energylosses.

FIG. 1 illustrates an exemplary pulse multiplier 100 configured togenerate pulse trains from each input pulse 101. Input pulse 101impinges on a polarizing beam splitter 102, which because of the inputpolarization of input pulse 101, transmits all of its light to a lens106. Thus, the transmitted polarization is parallel to the inputpolarization of input pulse 101. Lens 106 focuses and directs the lightof input pulse 101 to a half-wave plate 105. In general, a wave platecan shift the phases between perpendicular polarization components of alight wave. For example, a half-wave plate receiving linearly polarizedlight can generate two waves, one wave parallel to the optical axis andanother wave perpendicular to the optical axis. In half-wave plate 105,the parallel wave can propagate slightly slower than the perpendicularwave. Half-wave plate 105 is fabricated such that for light exiting, onewave is exactly half of a wavelength delayed (180 degrees) relative tothe other wave. Moreover, the combination of the two waves isorthogonally polarized compared to the light entering the plate.

Thus, half-wave plate 105 can generate pulse trains from each inputpulse 101. The normalized amplitudes of the pulse trains are: cos 2θ(wherein θ is the angle of half-wave plate 105), sin² 2θ, sin² 2θ cos2θ, sin² 2θ cos² 2θ, sin² 2θ cos³ 2θ, sin² 2θ cos⁴ 2θ, sin² 2θ cos⁵ 2θ,etc. Notably, the total energy of the pulse trains from a laser pulsecan be substantially conserved traversing half-wave plate 105.

The sum of the energy from the odd terms generated by half-wave plate105 is equal to:

(cos 2θ)²+(sin² 2θ cos 2θ)²+(sin² 2θ cos³ 2θ)²+(sin² 2θ cos⁵ 2θ)²+(sin²2θ cos⁷ 2θ)²+(sin² 2θ cos⁹ 2θ)²+ . . . =cos² 2θ sin⁴ 2θ(cos² 2θ+cos⁶2θ+cos¹⁰ 2θ+ . . . )=2 cos² 2θ/(1+cos² 2θ)

In contrast, the sum of the energy from the even terms generated byhalf-wave plate 105 is equal to:

(sin² 2θ)²+(sin² 2θ cos² 2θ)²+(sin² 2θ cos⁴ 2θ)²+(sin² 2θ cos⁶2θ)²+(sin² 2θ cos⁸ 2θ)²+(sin² 2θ cos¹⁰ 2θ)²+ . . . =sin⁴ 2θ(1+cos⁴2θ+cos⁸ 2θ+cos¹² 2θ+ . . . )=sin² 2θ/(1+cos² 2θ)

In accordance with one aspect of pulse multiplier 100, the angle θ ofhalf-wave plate 105 can be determined (as shown below) to provide thatthe odd term sum is equal to the even term sum.

2 cos² 2θ=sin² 2θ

cos² 2θ=⅓

sin² 2θ=⅔

θ=27.3678 degrees

Referring back to FIG. 1, the light exiting half-wave plate 105 isreflected by mirrors 104 and 103 back to polarizing beam splitter 102.Thus, polarizing beam splitter 102, lens 106, half-wave plate 105, andmirrors 104 and 103 form a ring cavity configuration. The lightimpinging on polarizing beam splitter 102 after traversing the ringcavity has two polarizations as generated by half-wave plate 105.Therefore, polarizing beam splitter 102 transmits some light andreflects other light, as indicated by arrows 109. Specifically,polarizing beam splitter 102 transmits the light from mirror 103 havingthe same polarization as input pulse 101. This transmitted light exitspulse multiplier 100 as output pulses 107. The reflected light, whichhas a polarization perpendicular to that of input pulse 101, isre-introduced into the ring cavity (pulses not shown for simplicity).

Notably, these re-introduced pulses can traverse the ring in the mannerdescribed above with further partial polarization switching by half-waveplate 105 and then light splitting by polarizing beam splitter 102.Thus, in general, the above-described ring cavity is configured to allowsome light to exit and the rest of the light (with some minimal losses)to continue around the ring. During each traversal of the ring (andwithout the introduction of additional input pulses), the energy of thetotal light decreases due to the light exiting the ring as output pulses107.

Periodically, a new input pulse 101 is provided to pulse multiplier 100.In one embodiment, for a 125 MHz laser input, 0.1 nanosecond (ns) laserpulses result. Note that the size of the ring, and thus the time delayof the ring, can be adjusted by moving mirror 104 along the axisindicated by arrows 108.

The ring cavity length may be slightly greater than, or slightly lessthan, the nominal length calculated directly from the pulse intervaldivided by the multiplication factor. This results in the pulses notarriving at exactly the same time as the polarized beam splitter andslightly broadens the output pulse. For example, when the input pulserepetition rate is 125 MHz, the cavity delay would nominally be 4 ns fora frequency multiplication by 2. In one embodiment, a cavity lengthcorresponding to 4.05 ns can be used so that the multiply reflectedpulses do not arrive at exactly the same time as an incoming pulse.Moreover, the 4.05 ns cavity length for the 125 MHz input pulserepetition rate can also advantageously broaden the pulse and reducepulse height. Other pulse multipliers having different input pulse ratescan have different cavity delays.

Notably, polarizing beam splitter 102 and half-wave plate 105 working incombination generate even and odd pulses, which diminish for each roundtraversed inside the ring. These even and odd pulses can becharacterized as providing energy envelopes, wherein an energy envelopeconsists of an even pulse train (i.e. a plurality of even pulses) or anodd pulse train (i.e. a plurality of odd pulses). In accordance with oneaspect of pulse multiplier 100, these energy envelopes are substantiallyequal.

FIG. 2A illustrates exemplary energy envelopes 202A, 202B, 202C, and202D, which consist of output pulse trains 201A, 201B, 201C, and 201D,respectively. As shown, output pulse trains exemplify theabove-described embodiment. That is, time delays between odd/even pulsesis 0.1 ns and time delays between associated pulses (i.e. 1→2, 3→4, 5→6)of adjacent power envelopes is 4.050 ns. Notably, the time betweenodd/even pulses is far enough apart so that they can be incoherentlyadded (and conversely that they do not coherently interfere with oneanother).

Note that original pulses 200A and 200B are not part of power envelopes202A and 200C, but are shown for context. Specifically, polarizing beamsplitter 102 and half-wave plate 105 use original pulses 200A and 200Bto generate output pulse trains 201A-201D. FIG. 2B illustrates that thenormalized sum of the individual pulses in each of pulse trains 201A and201B is equal to ½ and the normalized sum of pulse trains 201A and 201Bis equal to 1. Thus, the configuration described for pulse multiplier100 can double the original repetition pulse rate while reducing peakpower and ensuring energy balancing outputs.

Notably, referring back to FIG. 1, during each traversal of the ring,lens 106 can uniformly shape the light pulses. This uniformity allowspulses to be added (for example, as shown in FIG. 2B) with consistentresults of predetermined size envelopes (for example, as shown in FIG.2A). Thus, lens 106 can advantageously maintain high beam quality forpulse multiplier 100.

Note that although only one lens, i.e. lens 106, is shown in pulsemultiplier 100, other embodiments may include more lenses. The purposeof having at least one lens in the above-described pulse multiplier isto ensure uniform Gaussian beam shape at specific points in the beamrelay, i.e. to refocus the beam waist to compensate for the length ofthe ring cavity. FIGS. 3A, 3B, and 3C illustrate lens configurations for1 lens, 2 lenses, and 4 lenses, respectively. Note that the number oflenses refers specifically to the number of lenses in the ring cavity.Therefore, for example, configuration 301 (FIG. 3A) has one lens formingpart of the ring cavity, but in fact requires an additional two lensesoutside the ring cavity to form collimated beams. Note that horizontaland vertical lines in the Gaussian beam relays shown in FIGS. 3A-3Cindicate image planes, which is known to those skilled in the art,whereas diagonal lines refer to either mirrors or the polarizing beamsplitter. For example, in configuration 302 (FIG. 3B), three imageplanes 304 are provided. FIG. 3C illustrates a configuration 303 having4 lenses, which forms a telescopic pair having a magnification of 1×.Configuration 303 (like configuration 302) also generates two internalimages. However, configuration 303 does not require mirrors between thelens pair forming the telescope. Therefore, configuration 303 could bebuilt using two image relay tubes with adjustment mirrors between thetubes, thereby simplifying component alignment and component assemblycompared to configuration 302, for example.

Generally, a 2 lens configuration (also called a lens doublet) canprovide beam quality at the refocused beam waist than a 1 lensconfiguration. However, the number of lenses in the lens configurationmay vary based on the requirements of a specific application.Alternative pulse multiplier embodiments may include using one or morecurved focusing mirrors instead of, or in addition to, the one or morelenses. In one embodiment, the laser beam diameter is expanded to about10 mm wide before entering the ring cavity and therefore does not needrefocusing. In this embodiment having what can be characterized as awide beam, both lenses and curved mirrors can be eliminated.

FIGS. 4 and 5 illustrate how mirror tilt can affect output beams offset(in millimeters). Note that referring back to FIG. 1, the functionprovided by mirror 104 can also be performed using two mirrors 104A and104B, wherein mirror 104A can be characterized as a first corner mirror(when traversing the ring cavity) and mirror 104B can be characterizedas a second corner mirror. FIGS. 4 and 5 illustrate the sensitivity offirst and second corner mirrors, respectively, based on mirror tilt.Three lens configurations are shown: 1 lens configuration (401)(501), 2lens configuration (402)(502), and 4 lens configuration (403)(503). FIG.4 indicates that the 1 lens configuration has significantly moresensitivity to mirror tilt than the 2 or 4 lens configurations (whichare relatively close in sensitivity). FIG. 5 indicates that the 4 lensconfiguration significantly reduces sensitivity to mirror tilt comparedto either the 1 or 2 lens configuration.

Note that some advantages can be realized by using composite mirror 104rather than separate mirrors 104A and 104B. For example, pre-assembly ofcomposite mirror 104 to provide an exact 90 degree angle can facilitateeasier field assembly than aligning individual mirrors 104A and 104B.Moreover, composite mirror 104 can provide a return direction that isindependent of the angle of the two mirrors. Therefore, composite mirror104 can be rotated while still ensuring that light will always bereflected in parallel to input light. As a result, composite mirror 104may provide some performance advantages to separate mirrors 104A and104B. Composite mirror 104 can be implemented using reflecting prisms,glass blocks, machined mirrors, or other suitable materials.

FIG. 6 illustrates how lens tilt can affect output beams offset (inmillimeters). The sensitivities of four different lenses are shown inFIG. 6: 1 lens (601), 1^(st) of 2 lenses (602), 1^(st) of 4 lenses(603), and 2^(nd) of 4 lenses (604). As shown, a one lens configurationexhibits moderately more sensitivity to tilt than any otherconfiguration as the tilt angle increases.

FIG. 7 illustrates how lens decenter misalignment can affect outputbeams offset (both in millimeters). The sensitivities of four differentlenses are shown in FIG. 7: 1 lens (701), 1^(st) of 2 lenses (702)],1^(st) of 4 lenses (703), and 2^(nd) of 4 lenses (704). As shown, a lensconfiguration exhibits significantly more sensitivity to decentermisalignment than the 4 lens configuration (either 1^(st) or 2^(nd)lenses) and moderately more sensitivity to decenter misalignment thanthe 2 lens configuration.

Tables 1 and 2 provide exemplary data on how the beam splitterextinction ratio and polarization can affect energy efficiency for 2 and4 lenses. Note that Tables 1 and 2 assume (1) an input beam is inperfect P-polarization, (2) high-reflector (HR) coating mirrors are Rp:99.89%, Rs: 99.95%, (3) anti-reflective (AR) lenses are R: 0.2%), 4lenses (8 surfaces), (4) the first reflection is included in thecalculation, and (5) the half-wave plate is not fixed at 27.36 degrees.

The beam splitter extinction ratio is the ratio of the transmission ofthe wanted component to the unwanted component (i.e. for a polarizer,the ratio of the transmitted light to the reflected light). Notably, thepolarization purity is predominantly a function of the beam splitterextinction ratio. In one embodiment, an additional polarizer can beadded at the output of the pulse multiplier to improve polarizationpurity with a small loss.

The best angle for the half-wave plate to reach equal pulse-to-pulseenergy will depend on extinction ratio and other cavity losses. Tables 1and 2 consider examples using a finite extinction ratio polarizer andnon-ideal component transmissions and reflectivities, and estimate theoptimum waveplate angle requirement.

TABLE 1 Extinction Ratios for 2 Lenses Beamsplitter Extinction EnergyPolarization Half-Wave Ratio Tp/Ts Efficiency % Purity P/S Plate Angle100:1 96.21 99.73 26.85 150:1 97.3 150.27 26.29 200:1 97.6 200.4 26.15500:1 98.0 500.07 26.45

TABLE 2 Extinction Ratios for 4 Lenses Beamsplitter Extinction EnergyPolarization Half-Wave Ratio Tp/Ts Efficiency % Purity P/S Plate Angle100:1 95.16 99.93 26.35 150:1 95.12 148.68 27.45 200:1 96.4 200.6 26.05500:1 96.5 498.75 26.45

In one preferred embodiment, the number of components in the pulsemultiplier can be minimized. Specifically, for even small lossesassociated with each component, such as those shown in Table 1 above,each traversal of light through the ring cavity can minimally degradeperformance by a predetermined amount. Therefore, minimizing componentsin that ring cavity can provide one way of minimizing performancedegradation. For example, each lens has two surfaces, each surfacehaving a predetermined loss. Therefore, a 1- or 2-lens configuration(with 2 and 4 surfaces, respectively) may provide better performancethan a 4-lens configuration (with 8 surfaces) (assuming lenses ofequivalent quality).

FIG. 8 illustrates an exemplary embodiment of a pulse multiplier 900including two lenses 801A and 801B. Note also that mirror 104 (FIG. 1)is now separate mirrors 104A and 104B. In this embodiment, lens 801A ispositioned between polarizing beam splitter 102 and mirror 104A, whereaslens 801B, half-wave plate 105 and mirror 103 are positioned betweenmirror 104B and polarizing beam splitter 102. This configuration and anyothers having one ring cavity and any number of lenses can provide a 2×rate increase in output pulses 107 compared to the rate of input pulse101.

In one embodiment, two cavities of different lengths can be used inseries to multiply the pulse rate by four or more. For example, FIG. 9Aillustrates a pulse multiplier including two ring cavities 900A and 900Bin series (note that ring cavities 900A and 900B could be adjacent onthe same plane, as shown, or place one on top of another), each ringcavity including a polarizing beam splitter 901, a half-wave plate 902,and mirrors 903 (lens or lenses not shown for simplicity).

FIG. 9B illustrates a pulse multiplier including a semi-nested ringcavity, thereby allowing the sharing of some components between two ringcavities. For example, in this embodiment, polarizing beam splitter 911,half-wave plate 912, and mirror 913′ can be shared by both ring cavities910A and 910B (ring cavity 910B having two portions, one portion nestedwithin ring cavity 910A and the other portion outside ring cavity 910A).Note that mirrors 913 form part of their respective ring cavities (lensor lenses, which can be placed using conventional practice, not shownfor simplicity). As shown in FIG. 9B, after the light leaves ring cavity910A, the light first traverses the portion of ring cavity 910B outsidering cavity 910A, then traverses the portion of ring cavity 910 nestedin ring cavity 910A. In one embodiment, the second ring cavity 900B/910Bcan have substantially half the cavity length of the first ring cavity(900A/910A) to provide a pulse repetition rate multiplied by four.

Notably, the pulse multipliers in FIGS. 9A and 9B can provide a 4× rateincrease compared to the input pulse. Other embodiments can include morering cavities, wherein each ring increases the rate (e.g. 3 ringcavities provides 8×, 4 ring cavities provides 16×, etc.).

Note that although a half-wave plate is included in the above-describedpulse multiplier embodiments, other wave plates can be used in otherembodiments. That is, one or more wave plates of different retardancesmay be used instead of a single half-wave plate. For example, ahalf-wave plate can be replaced by a quarter-wave plate or a combinationof a half-wave and a quarter-wave plate depending on the desiredmultiplication factor and whether a train of equal strength pulses isrequired or if a train of decaying pulse amplitudes is required.

In one embodiment of a pulse multiplier, at least one ring cavity caninclude 2 wave plates. In this case, the first wave plate can provide aphase delay of δ1 at angle θ1 and the second wave plate can provide aphase delay of δ2 at angle θ2. The electric field of the input laserpulse (E_(x), E_(y)) can be determined by:

$\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = {\begin{bmatrix}{{\cos^{2}\theta_{2}} + {^{{\delta}_{2}}\sin^{2}\theta_{2}}} & {( {1 - e^{{\delta}_{2}}} )\sin \; \theta_{2}\cos \; \theta_{2}} \\{( {1 - e^{{\delta}_{2}}} )\sin \; \theta_{2}\cos \; \theta_{2}} & {{\sin^{2}\theta_{2}} + {^{{\delta}_{2}}\cos^{2}\theta_{2}}}\end{bmatrix}{\quad{\begin{bmatrix}{{\cos^{2}\theta_{2}} + {^{{\delta}_{2}}\sin^{2}\theta_{2}}} & {( {1 - e^{{\delta}_{2}}} )\sin \; \theta_{2}\cos \; \theta_{1}} \\{( {1 - e^{{\delta}_{2}}} )\sin \; \theta_{2}\cos \; \theta_{2}} & {{\sin^{2}\theta_{2}} + {^{{\delta}_{2}}\cos^{2}\theta_{1}}}\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}}}$

In one embodiment, the first phase plate can be set as a quarter-waveplate and the second phase plate as a half-wave plate, as indicatedbelow.

$\begin{bmatrix}E_{x}^{\prime} \\E_{y}^{\prime}\end{bmatrix} = {\begin{bmatrix}{\cos^{2}\theta_{2}} & {\sin \; \theta_{2}} \\{\sin \; \theta_{2}} & {{- \cos^{2}}\theta_{2}}\end{bmatrix}{\quad{\begin{bmatrix}{{\cos^{2}\theta_{1}} + {{sin}^{2}\theta_{1}}} & {( {1 - } )\sin \; \theta_{1}\cos \; \theta_{1}} \\{( {1 - } )\sin \; \theta_{1}\cos \; \theta_{1}} & {{\sin^{2}\theta_{1}} + {{cos}^{2}\theta_{1}}}\end{bmatrix}\begin{bmatrix}E_{x} \\E_{y}\end{bmatrix}}}}$

In one embodiment, the ring cavity can be aligned as described below.Initially, the pulse shape and timing can be observed using a photodiodeand an oscilloscope to adjust the cavity length. Then, a camera located1-2 m from the ring cavity exit can be used to detect the laser beamprofile and location. At this time, the wave plate θ can be set to zerodegrees. In this configuration, the pulse goes once around the ringcavity and exits with no significant reflected light from the polarizedbeam splitter being redirected to the ring cavity (and thus no rateincrease should occur). Then the wave plate θ can be set to 45 degrees.In this configuration, the pulse should traverse the ring cavity twiceand then exit. Specifically, insignificant transmission occurs throughthe polarized beam splitter after the first pass, but substantiallycomplete transmission occurs after the second pass. Finally, the waveplate θ can be set to 27.3678 degrees so that the even and odd pulsesenergies will reach a balanced average power spatially upon exit fromthe cavity. Then, the optical components can be adjusted to ensure thesetwo trace of light paths are of substantially the same size and arriveat the same location.

Moreover, pulse multipliers with ring cavities are capable of generatingpulse trains with different amplitudes, should that feature be desired.For example, with an appropriate wave plate orientation, the secondpulse could be stronger than the first pulse. Specifically, if the axisof the half-wave plate is oriented at an angle to the x axis (the planecontaining the polarization vector of the incoming laser) greater thanabout 27.4 degrees, then the first pulse will be weaker than the secondpulse. Alternative configurations can divide one pulse into a train ofpulses of decreasing amplitude. Such a train could then repeat for eachincoming laser pulse.

Although pulse multipliers including ring cavities are described above,other pulse multiplier may include multi-surface reflection schemeswithout a ring cavity for generating pulses. For example, FIG. 10illustrates a pulse multiplier 1000 including a polarizing beam splitter1001, a quarter-wave plate 1002, a mirror 1003, and two etalon-likesurfaces 1004. An etalon is typically formed by a transparent platehaving two highly reflecting surfaces. In this embodiment, etalon-likesurfaces 1004 can be formed with partially reflective surfaces. Notethat in pulse multiplier 1000, the alignment of the optical components(and associated multiple surfaces) is relatively simple, although aninterferometer may be needed for accurate alignment. Table 3 belowindicates the exemplary reflectance R for various numbers (n) ofsurfaces. In general, optimized results are generated when (1−R)^(2n)=R,where R is the reflectance.

TABLE 3 Number of Surfaces vs. Reflection n R 1 0.382 2 0.276 3 0.222 40.188

FIG. 11 illustrates an exemplary pulse multiplier 1100 that uses twocombined beams to generate pulse outputs. Pulse multiplier 1100 includesa polarizing beam splitter 1101, two mirrors 1103 and 1106, twoquarter-wave plates 1102 and 1105, and a half-wave plate 1104. In thisconfiguration, polarizing beam splitter 1101 can direct light to bothmirrors 1103 and 1106 via quarter-wave plates 1102 and 1105,respectively. The reflected light from mirrors 1103 and 1106 having thesame polarization (again passing through quarter-wave plates 1105 and1102) can be combined using polarizing beam splitter 1101. Pulsemultiplier 1100 can only provide double the repetition rate of the inputpulse, as shown. Note that for this configuration, an interferometer canbe used to align pulse multiplier 1100.

FIG. 12 illustrates an exemplary pulse multiplier 1200 that reduces thenumber of mirrors in the ring cavity (e.g. compared to FIG. 1). Pulsemultiplier 1200 includes a triangular ring cavity having a polarizingbeam splitter 1201, two mirrors 1203 and 1204, and a half-wave plate1202. In the configuration shown in FIG. 12, the first pulse isreflected from polarizing beam splitter 1201 and has a firstpolarization, whereas the second pulse is transmitted and has a secondpolarization different than the first polarization. The peak outputpower is tunable to sin² θ. Note that for this configuration, aninterferometer can be used to align pulse multiplier 1200.

Advantageously, inspection systems can include the above-described pulsemultipliers. The inspection system can be a bright-field inspectionsystem, a dark-field inspection system, or a system with bothbright-field and dark-field modes. The inspection system can beconfigured to inspect semiconductor wafers or photo-lithography masks.Specifically, the inspection system may be configured to detectpatterning defects on a patterned sample, or may be configured to detectparticles, pits, or bumps on a patterned or un-patterned surface.

For example, the high-repetition rate laser pulses generated by theabove-described pulse multipliers can be used in a flash-on-the-flyinspection system, wherein a single laser pulse illuminates a portion ofa moving sample (such as a wafer or reticle) that is to be inspected andan image is acquired by a camera. Because each laser pulse is of shortduration, the motion is effectively frozen and an un-blurred image isacquired. Advantageously, a higher repetition rate, as provided by theabove-described pulse multipliers, can enable more images to be acquiredper unit time, thereby allowing faster motion.

FIG. 13 illustrates an exemplary wafer inspection system 1300 includinga pulse multiplier 1320. In system 1300, a waver 1301 can be rotated andtranslated using a mechanism 1302 to ensure the wafer's whole surface isscannable. Pulse multiplier 1302 can advantageously generate pulses fora normal beam 1303 and an oblique beam 1304 that are directed onto wafer1301. The reflected incident light from wafer 1301 is then directed, forexample using a Coblenz sphere 1308 and optics 1309, onto detectors (notshown for simplicity). System 1300 can provide both narrow and widedetection paths, e.g. including a narrow photo multiplier tube (PMT) 105and a wide PMT 1306. U.S. Pat. No. 5,189,481, which issued to Jann etal. on Feb. 23, 1993, describes system 1300 in greater detail, and isincorporated by reference herein. Notably, pulse multiplier 1320 canmultiply the pulses from a UV, DUV, or VUV laser. Pulse multiplier 1320can advantageously increase the repetition rate while reducing the peakpower of whatever laser is used.

FIG. 14 illustrates an exemplary patterned wafer inspection system 1400including a pulse multiplier 1401, which can provide both near-normaland oblique illumination (only oblique illumination 1402 shown forclarity). Pulse multiplier 1401 can generates pulses from a UV, DUV, orVUV laser. Advantageously, pulse multiplier 1401 can increase therepetition rate of the laser used, while reducing its peak power. Insystem 1400, multi-channel collection 1403 can provide a largecollection area, binning, and channel fusion with an increased signal tonoise ratio (SNR). Illumination polarization, as generated by pulsemultiplier 1401, can provide previous layer suppression and defectselectivity. The illumination channels, which facilitate multi-channelcollection 1403, can illuminate one or more spots, one or more narrowlines, or a rectangular area on wafer 1404. Detection channels caninclude Fourier filtering (for pattern suppression), polarizationselection, angle range, and/or numerical aperture (NA) control.

Advantageously, metrology systems can also include the above-describedpulse multipliers. Exemplary metrology systems can include, but are notlimited to, an ellipsometer (see, e.g. U.S. Pat. No. 6,734,968,incorporated by reference herein), an angle-resolved reflectometer (see,e.g. U.S. Pat. No. 4,999,014 or U.S. Pat. No. 7,667,841, bothincorporated by reference herein) or a photo-acoustic measurement system(see, e.g. U.S. Pat. No. 4,710,030, incorporated by reference herein).

Note that any inspection or metrology system including a pulsemultiplier can be used in combination with a pulse-shaping device.Exemplary pulse-shaping devices include but are not limited to thosedescribed in U.S. patent application Ser. No. 13/061,936 filed Mar. 2,2011, which is a National Stage application of PCT Published ApplicationWO2010/037106, which claims priority of U.S. Provisional Application61/100,990, all applications being incorporated by reference herein.Such pulse-shaping devices can be used to reduce the coherence of eachlaser pulse or otherwise modify the shape of the pulse.

FIG. 15 illustrates an exemplary pulse multiplier 1500 including mirrorsand at least one half-wave plate In this embodiment, pulse multiplier1500 can include a polarizing beam splitter 1501, which directs theincoming light to a flat mirror 1502. In one embodiment, a half-waveplate (not shown for simplicity) is positioned between polarizing beamsplitter 1501 and mirror 1502. In other embodiments, one or morehalf-wave plates can be positioned as shown in other pulse multiplierembodiments described above.

Flat mirror 1502 reflects the light to a first spherical mirror 1503,which in turn directs the light to a second spherical mirror 1504.Second spherical mirror 1504 then directs the light back to firstspherical mirror 1503, which in turn directs the light throughpolarizing beam splitter 1501. In one embodiment, first spherical mirror1503 can have a 2× radius of second spherical mirror 1504. Note thatfirst spherical mirror 1503 and second spherical mirror 1504 can bepositioned to parallel positions, wherein the decenter of firstspherical mirror 1503 can determine the relative position of secondspherical mirror 1504. As shown in FIG. 15, polarizing beam splitter1501 can have a Brewster angle, although in other embodiments,polarizing beam splitter 1501 can have a 45 degree angle. In oneembodiment, the mirrors used in pulse multiplier 1500 can be mounted ona rail or in a tube for a compact and stable design. The practicalconvenience of this implementation is a notable feature. Note thatgeometric aberrations can be reduced to much lower than the diffractionlimit by limiting the light beam diameter to, for example, less than afew mm extent, depending on laser beam quality requirements. In oneembodiment, an optional polarizing beam splitter 1505 can be included atthe output of pulse multiplier 1500 to improve polarization contrast.

Notably, the pulse multiplier can inexpensively reduce the peak powerper pulse while increasing the number of pulses per second with minimaltotal power loss. The pulse multiplier can advantageously enable highspeed inspection and metrology with off-the-shelf lasers. Dark-fieldinspection systems rely on laser light sources. The above-describedpulse multiplier allows those systems to use lasers that would otherwisehave too low a pulse repetition rate and provides a potentialalternative to extremely high repetition rate UV lasers or CW lasers ifno appropriate laser is available, or available lasers are too expensiveor unreliable.

A detailed description of one or more embodiments of the invention isprovided above along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment.

For example, in one embodiment, the optical components can be coatedwith appropriate coatings for the laser wavelength. Each surface of thetransmission elements, i.e. lens(es), and waveplate(s), can also have ananti-reflection coating that minimizes the amount of laser energyreflected at each surface. The mirrors can be polished and coated with acoatings designed to maximize the reflection and minimize scattering atthe laser wavelength.

Note that a ring cavity can also be implemented using reflective opticsas shown by ring cavity 1600, shown in FIG. 16. In this embodiment, theimage relay can be facilitated by beamsplitter 1601, mirror 1602,spherical lens 1603, and spherical lens 1604. The principle is much thesame as with the above-described lens systems, but with fewerinteracting surfaces and potentially less loss.

FIG. 17 illustrates another exemplary pulse multiplier 1700 configuredto generate pulse trains from each input pulse. An input pulse impingeson a non-polarizing beam splitter 1701, which transmits half of itslight to a non-polarizing beam splitter 1702 and reflects the remaininghalf of its light to a mirror 1703. Mirror 1703 reflects that light tomirror 1704, which in turn reflects that light to non-polarizing beamsplitter 1702. The total distance that the light reflected by beamsplitter 1701 travels before arriving at beam splitter 1702 is chosensuch that it introduces delay equals to the inverse of double therepetition rate of the incident light. In turn, non-polarizing beamsplitter 1702 transmits half and reflects half of its light receivedfrom each of non-polarizing beam splitter 1701 and mirror 1704,therefore producing two beams with doubled pulse repetition rates. Ahalf-wave plate 1705 and a mirror 1707 receive the light transmitted andreflected by non-polarizer beam splitter 1702. A mirror 1706 receivesthe two waves generated by half-wave plate 1705. A polarizing beamsplitter 1708 receives the reflected light from both mirrors 1706 and1707. Polarizing beam splitter 1708 combines the in-phase light andgenerates a forty-five degree angle output polarization for the pulsetrain (i.e. a repetition rate doubling scheme).

FIG. 18 illustrates another exemplary pulse multiplier 1800 configuredto generate pulse trains from each input pulse. An input pulse impingeson a non-polarizing beam splitter 1801, which transmits half of itslight to a non-polarizing beam splitter 1803 and reflects the remaininghalf of its light to a mirror 1802. Mirror 1802 reflects that light tomirror 1803, which in turn reflects the light to non-polarizing beamsplitter 1803. Non-polarizing beam splitter 1803 splits the receivedlight between a mirror 1805 and a non-polarizing beam splitter 1807.Mirror 1805 reflects its light to a mirror 1806, which in turn reflectsthe light to non-polarizing beam splitter 1807. Notably, pulsemultiplier 1800 includes N steps, wherein each step includes theabove-described non-polarizing beam splitters and mirrors and results in2× increase in pulse repetition rate to a total of 2^(N)× after N steps.A non-polarizing beam splitter 1808 receives the transmitted light fromthe last step and splits the light between a mirror 1809 and a half-waveplate 1810. A mirror 1811 reflects the light output by half-wave plate1810 to a polarizing beam splitter 1812, which also receives reflectedlight from mirror 1809. Polarizing beam splitter 1812 generates lightsat a 45 degree output polarization.

The scope of the invention is limited only by the claims and theinvention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the abovedescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

1. A pulse multiplier comprising: a polarizing beam splitter forreceiving an input laser pulse; a wave plate for receiving light fromthe polarized beam splitter and generating a first set of pulses and asecond set of pulses, the first set of pulses having a differentpolarization than the second set of pulses; and a set of mirrors forcreating a ring cavity including the polarizing beam splitter and thewave plate, wherein the polarizing beam splitter transmits the first setof pulses as an output of the pulse multiplier and reflects the secondset of pulses into the ring cavity.
 2. The pulse multiplier of claim 1,wherein the wave plate includes a half-wave plate.
 3. The pulsemultiplier of claim 2, wherein the half-wave plate is set at 27.3678degrees.
 4. The pulse multiplier of claim 1, wherein the wave plateincludes a quarter-wave plate.
 5. The pulse multiplier of claim 1,further including a lens for uniformly shaping pulses in the ringcavity.
 6. The pulse multiplier of claim 1, further including aplurality of lenses for uniformly shaping pulses in the ring cavity. 7.The pulse multiplier of claim 6, wherein the plurality of lenses isimplemented with two image relay tubes.
 8. The pulse multiplier of claim6, wherein the plurality of lenses consists of two or four lenses. 9.The pulse multiplier of claim 1, wherein the mirror set includes acomposite mirror.
 10. The pulse multiplier of claim 1, wherein themirror set creates two ring cavities that share the polarizing beamsplitter and the wave plate.
 11. A pulse multiplier comprising: a firstring cavity including: a first polarizing beam splitter; a first waveplate for receiving light from the first polarized beam splitter andgenerating a first set of pulses and a second set of pulses, the firstset of pulses having a different polarization than the second set ofpulses; a first set of mirrors for creating the first ring cavityincluding the first polarizing beam splitter and the first wave plate;and a second ring cavity including: a second polarizing beam splitter; asecond wave plate for receiving light from the first polarized beamsplitter and generating a third set of pulses and a fourth set ofpulses, the third set of pulses having a different polarization than thefourth set of pulses; a second set of mirrors for creating the secondring cavity including the second polarizing beam splitter and the secondwave plate wherein the first polarizing beam splitter transmits an inputlaser pulse into the first ring cavity, transmits the first set ofpulses into the second ring cavity, and reflects the second set ofpulses into the first ring cavity, and wherein the second polarizingbeam splitter transmits the second set of pulses into the second ringcavity, transmits the third set of pulses as outputs of the pulsemultiplier and reflects the fourth set of pulses into the second ringcavity.
 12. A pulse multiplier comprising: a polarizing beam splitterfor receiving an input laser pulse; a wave plate for receiving lightfrom the polarizing beam splitter and generating a first set of pulsesand a second set of pulses, the first set of pulses having a differentpolarization than the second set of pulses; a set of multi-surfacereflecting components for reflecting the first and second sets of pulsesback through the wave plate to the polarizing beam splitter, wherein thepolarizing beam splitter transmits the first set of pulses as an outputof the pulse multiplier and reflects the second set of pulses back tothe wave plate and the set of multi-surface reflecting components. 13.The pulse multiplier of claim 12, wherein the wave plate includes aquarter-wave plate.
 14. The pulse multiplier of claim 12, wherein themulti-surface reflecting components include a mirror and two etalons.15. The pulse multiplier of claim 12, wherein a peak output power of thesecond set of pulses is tunable to sin² θ.
 16. A pulse multipliercomprising: a first wave plate for receiving an input laser pulse; apolarizing beam splitter for receiving outputs of the first wave plate;a second wave plate for receiving a first set of pulses from thepolarizing beam splitter; a first mirror for reflecting outputs from thesecond wave plate back through the second wave plate to the polarizingbeam splitter; a third wave plate for receiving a second set of pulsesfrom the polarizing beam splitter; a second mirror for reflectingoutputs from the third wave plate back through the third wave plate tothe polarizing beam splitter, wherein the polarizing beam splittertransmits a third set of pulses from the second wave plate combined witha fourth set of pulses from the third wave plate to generate an outputof the pulse multiplier, reflects a fifth set of pulses from the secondwave plate back to the second wave plate and the first mirror, andreflects a sixth set of pulses back to the third wave plate and thesecond mirror.
 17. The pulse multiplier of claim 16, wherein the firstwave plate includes a half-wave plate.
 18. The pulse multiplier of claim16, wherein the second and third wave plates include quarter-waveplates.
 19. A system comprising: a pulse multiplier including: apolarizing beam splitter for receiving an input laser pulse; a waveplate for receiving light from the polarized beam splitter andgenerating a first set of pulses and a second set of pulses, the firstset of pulses having a different polarization than the second set ofpulses; a set of mirrors for creating a ring cavity including thepolarizing beam splitter and the wave plate, wherein the polarizing beamsplitter transmits the first set of pulses as an output of the pulsemultiplier and reflects the second set of pulses into the ring cavity.20. The system of claim 19, wherein the system implements one of anunpatterned wafer inspection system, a patterned wafer inspectionsystem, a mask inspection system, and a metrology system.
 21. A methodof generating pulses for a system, the method comprising: opticallysplitting an input laser pulse into a plurality of pulses using a ringcavity including a polarizing beam splitter and a wave plate; groupingthe plurality of pulses into pulse trains, wherein the pulse trains areof approximately equal energy and are approximately equally spaced intime; and transmitting a set of the pulse trains as the pulses for thesystem and reflecting a remainder of the pulse trains into the ringcavity.
 22. A pulse multiplier comprising: a first polarizing beamsplitter for receiving incoming light; a half-wave plate for receivingpolarized light from the first polarizing beam splitter; a flat mirrorfor reflecting pulsed light from the half-wave plate; a first sphericalmirror for receiving reflected light from the flat mirror; a secondspherical mirror for receiving first directed light from the firstspherical mirror, the first spherical mirror also for receiving seconddirected light from the second spherical mirror, the first beam splitteralso for receiving third directed light from the first spherical mirrorand for generating output pulsed light.
 23. The pulse multiplier ofclaim 22, wherein the first spherical mirror has a 2× radius of thesecond spherical mirror.
 24. The pulse multiplier of claim 22, whereinthe first spherical mirror and the second spherical mirror are inparallel positions, and wherein a decenter of the first spherical mirrordetermines a relative position of the second spherical mirror.
 25. Thepulse multiplier of claim 22, further including a second polarizing beamsplitter for receiving the output pulsed light from the first polarizingbeam splitter and for improving polarization contrast.
 26. A pulsemultiplier comprising: a first non-polarizing beam splitter forreceiving incoming light; a second non-polarizing beam splitter forreceiving a first half of the light from the first non-polarizing beamsplitter; a first mirror for receiving a second half of the light fromthe first non-polarizing beam splitter; a second mirror for receivingreflected light from the first mirror, the second non-polarizing beamsplitter also for receiving reflected light from the second mirror,wherein a total distance traveled by light reflected by the firstnon-polarizing beam splitter and the first and second mirrors to thesecond non-polarizing beam splitter introduces a delay equal to aninverse of double a repetition rate of the incoming light; a half-waveplate for receiving a first half of output light from the secondnon-polarizing beam splitter; a third mirror for receiving a second halfof the output light from the second non-polarizing beam splitter; afourth mirror for receiving waves generated by the half-wave plate; anda polarizing beam splitter for receiving reflected light from the thirdand fourth mirrors, and generating a forty-five degree angle outputpolarization for a pulse train.